When a color image is compressed in conventional color image processing, compression is carried out by the use of human visual characteristics in that humans have poor color resolution capabilities. Consequently, the input image has the color space transformed, converted into a luminance signal and chrominance signal, and the amount of information in the chrominance signal is reduced. Examples of the kind of color spaces used for compression include YUV, L*a*b, etc. Requests have been increased for a compression technique that can achieve optimum compression, whichever color space is chosen from these color spaces.
For these requests, as disclosed in, for example, Japanese Patent Application, Jpn. Pat. Appln. KOKAI Publication No. 7-203211, a technique for compressing with parameters optimized for each color space is considered. Specifically, it is a method for preparing compression parameters on the compression side in compliance with various color spaces and carrying out compression using parameters of the color spaces chosen, and carrying out decompression on the decompression side in compliance with the color spaces on the compression side. It is a method for choosing on the compression side the color spaces used on the decompression side and then carrying out compression.
In the conventional compression method described above, it is a system for optimizing compression parameters in the color space only and carrying out compression by the use of one common parameter for individual colors on the color space. Consequently, by the characteristics specific to input apparatuses such as scanners, etc. and those specific to output apparatuses such as printers, etc., the characteristics specific to the apparatus are emphasized on the specific colors at the time of compression and decompression, and degradation of the screen quality may occur. Similarly, in the treatment after compression and decompression, the color is susceptible to characteristics specific to the apparatus and the specific colors may be poorly reproduced. Even in such an event, since the parameter used for compression and decompression is one, there is a problem in that the color cannot be effectively corrected for the specific color with poor reproducibility only.